The family of polysulfone polymers is well known in the art and three types of polysulfone have been available commercially viz. Polysulfone (PSU), Polyether Sulfone (PES) and Polyphenylene Sulfone (PPSU).
The commercially available Polysulfones (PSU, PPSU, PES) have good high temperature resistance and generally do not degrade or discolor at their processing temperatures of 350° C. to 400° C. Additionally, they are transparent, light amber colored amorphous plastics with excellent mechanical and electrical properties, and good chemical and flame resistance. These Polysulfones are readily processible using common plastics processing techniques such as injection molding, compression molding, blow molding and extrusion. This makes them very versatile and useful plastics, having a myriad of applications in electronics, the electrical industry, medicine, general engineering, food processing and other industries.
The Polysulfone PSU was discovered in early 1960 at Union Carbide (U.S. Pat. No. 4,108,837, 1978). Since then, activity in improving the quality of PSU has remained strong and improvements in color, thermal stability, molecular weights and reduction in residual monomer and solvent are continuously sought.
While, there are many similarities among PSU, PES, and PPSU as regards color, electrical properties, chemical resistance, flame resistance etc., there are also important differences. The foremost difference among these is the Glass Transition Temperature (Tg). PSU has a Tg of 189° C., PES has a Tg of 225° C., while PPSU has a Tg of 222° C. Thus, PSU has a lower overall thermal resistance in terms of its dimensional stability compared to PPSU and especially PES, which has the highest thermal resistance. Besides this, PES also has a higher tensile strength (>90 MPa) compared to PSU and PPSU (both 70-75 MPa). On the other hand, PPSU, like Polycarbonate (PC), has an outstanding impact resistance, and its Izod notched impact strength is 670-700 J/m. Both PES and PSU have lower Izod notched impact strengths of only 50-55 J/m. Similarly, it is known in the art that articles made from PPSU can withstand >1000 sterilization cycles without crazing, while PSU based articles withstand about 80 cycles and PES based articles withstand only about 100 cycles. PSU, on the other hand, has the lightest color and can be more readily processed, while PPSU is darker and more difficult to process than either PSU or PES.
Thus, a combination of PSU properties such as easy processibility and light color properties with PPSU properties such as high temperature and impact resistance would be desirable. Incorporating a proportion of PSU into PPSU may also bring down the overall cost. Although the physical blending of PPSU and PSU is one way of accomplishing this, it destroys one of the most important properties of both the homopolymers, namely their transparency. Similarly, a physical blend of PES and PSU is not only opaque, but also cannot be processed to give blends with desirable properties since they are very incompatible polymers.
Other polysulfone combinations, as discussed later, are also desirable as they give higher Tg's than that of PES, further boosting the high temperature resistance of these polymers by incorporating these units and making them more readily processible by incorporating PES, PSU or PPSU into their chain structures.
The unit chain structures of the part of family of polysulfones are given below:
PPSU:—C6H4—SO2—C6H4—O—C6H4—C6H4—O—PSU:—C6H4—SO2—C6H4—O—C6H4—C(CH3)2—O—PES:—C6H4—SO2—C6H4—O—C6H4—SO2—C6H4—O—PSS:—C6H4—C6H4—O—C6H4—SO2—C6H4—C6H4—SO2—C6H4—O—
The representative polysulfones shown above are prepared using one or more aromatic Dihalo compound such as Dichlorodiphenyl sulfone (DCDPS) or Dichlorodiphenyl disulfonylbiphenyl (CSB) and one or more of aromatic di-hydroxy monomer such as Bisphenol A, Dihydroxy diphenylsulfone (DHDPS), Biphenol, Dihydroxy diphenyl ether, Dihydroxy diphenyl methane, or their respective mono, di or tetra substituted Methyl derivatives, etc.
For PPSU, the di-hydroxy compound used is Biphenol (HO—C6H4—C6H4—OH), for PES, it is DHDPS (—HO—C6H4—SO2—C6H4—OH) and for PSU, it is Bisphenol A (HO—C6H4—C(CH3)2—C6H4—OH), while DCDPS (Cl—C6H4—SO2—C6H4—Cl) is used as the aromatic dihalo compound for all three of these commercially available polysulfones.
The use of more than one dihydroxy monomers is also known. For example, “PAS”, polyaryl sulfone, manufactured by Amoco is known to include a small quantity of hydroquinone in addition to DCDPS and DHDPS. The third monomer is added at the start of the manufacturing process and so gets polymerized in a random sequence in the polymer chain.
Other random copolymers in the prior art have shown that a third monomer may be added in much larger quantities. Thus, GB Patent 4,331,798 (1982) and U.S. Pat. No. 5,326,834 (1994) teach the preparation of terpolymers using 80-40 mole % of DHDPS and correspondingly 20-60 mole % of Biphenol with equivalent mole % of DCDPS. Since both patents teach that polymerization is to be started with the monomers themselves, it can be seen that the distribution of DHDPS and Biphenol in the final copolymer will be at random. Thus, one gets a random sequence such as: -ABAABBBAABAAABBABABBAAABB-, where A and B are present in a random sequence and in variable amounts depending upon the initial concentrations of A and B or DHDPS and Biphenol. The DCDPS moiety will be present in between A-A, A-B & B-B groups, although not shown here. Similarly, European Patent No. 0,331,492 teaches the synthesis of random terpolymers of DCDPS and DHDPS/Biphenol or Bisphenol A/Biphenol. The synthesis starts with three monomers and gives random terpolymers (and not block copolymers) in which the sequence of A & B in the chains cannot be predicted.
The prior art shows that block copolymers have been prepared where only one of the blocks is polysulfone. Hedtmann-Rein and Heinz (U.S. Pat. No. 5,036,146-1991) teach the preparation of a block copolymer of PSU with a polyimide (PI). In this case, a homoblock of an amine terminated polysulfone was prepared first. This was preformed using DCDPS, Bisphenol A and p-aminophenol to give a homoblock having a molecular weight in the range of 1500 to 20000. The homoblock produced was subsequently reacted with a tetracarboxylic acid, such as benzophenonetetracarboxylic dianhydride, and another diamine, such as 4,4′-diaminodiphenylmethane, to make a block copolymer of PSU-PI. The copolymers were prepared in the melt phase at 350° C.
McGrath and coworkers (Polymer preprints, 25, 14, 1984) have prepared PSU-Polyterphthalate copolymers. This was done using DCDPS (0.141 mole) and a mixture of hydroquinone and biphenol (0.075 mole each) to give a homoblock in solution, and then reacting the homoblock with a terephthaloyl chloride and biphenol, using solution or interfacial techniques, to give a block copolymer.
McGrath et al (Polymer Preprints, 26, 275, 1985) have further described preparations using acetyl end capped PSU with p-acetoxy benzoic acid or biphenol diacetate/terephthalic acid to obtain block copolymers of PSU/Polyethers, the latter part being highly crystalline or even liquid crystalline polymers. The synthesis of the block copolymer was carried out as a melt or in the presence of diphenyl sulfone at 200-300° C. Block copolymer preparation was indicated by the fact that the product was not soluble in common organic solvents.
McGrath and coworkers (Polymer Preprints, 26, 277, 1985) have also developed block copolymers of PSU and PEEK using a hydroxy terminated oligomeric PSU homoblock and difluoro benzophenone alone or optionally adding hydroquinone and/or biphenol. The first method, rather than giving a block copolymer, gives PSU blocks joined by difluorobenzophenone. However, the second method has the possibility of producing both random and block structures in the copolymers of PSU and PEEK.
While the above investigations have prepared PSU block copolymers, it can be seen that most have opted for the combination of hydroxy terminated PSU with other monomers, which on polymerization give block copolymers. In this process, it is quite likely that the polymerizing monomers would give block sizes so varied that some of the PSU blocks may be joined by nothing more than a single monomer unit having a molecular weight of only 300 or less, and certainly <1000. Thus, the molecular weight of the second block will not be that of a PSU oligomer, which should ideally be >1000 to be called a block. Thus, depending on the concentration, it is likely that the second homoblock will be no more than a single or double monomer unit. (sentence deleted)
Noshay and coworkers (J. Polymer Sci. A-1, 3147, 1971) have prepared block copolymers of amine terminated dimethyl Siloxanes and hydroxy terminated PSU. The hydroxy terminated PSU was prepared using a slight excess of Bisphenol A (0.495 mole) over DCDPS (0.450 mole). The —ONa groups were then converted to —OH groups using oxalic acid and the product was precipitated. The dried PSU powder was reacted with a separately prepared amine terminated polysiloxane in ether at 60° C. It may be noted that while PSU is plastic, Polysiloxane is elastomeric and hence the combination gives a block copolymer with thermoplastic elastomer like properties.
Surprisingly however, there has been no described method, nor synthesis carried out, whereby two Sulfone homoblocks have been used to form a block copolymer with thermoplastic properties.
The usual method of preparation of these polysulfones consists of the following process:
An aprotic organic solvent selected usually from Sulfolane, N-methyl pyrrolidone (NMP), Dimethyl Acetamide (DMAc), Diphenyl sulfone, Dimethyl sulfone or Dimethyl sulfoxide (DMSO), usually distilled over an alkali, is placed in the reactor. DCDPS or a similar dihalo monomer and the second dihydroxy monomer (Bisphenol A or Biphenol, etc.), generally in a molar proportion of 1.00:1.00, are added to this reactor along with sodium or potassium carbonate. Toluene or monochlorobenzene (MCB) is added to facilitate dehydration. The temperature of the mixture is then slowly increased from RT to 140° C. to 200° C. depending on the solvent utilized, whereupon the alkaline carbonate reacts with the phenol to give a salt and liberate water. The water gets distilled off, which is facilitated by toluene or MCB, if present.
The reaction mixture after water removal is then heated to a temperature in the range of 170° C. to 230° C., depending on the solvent, alkali and the dihydroxy monomer used, until the desired viscosity or molecular weight is attained. Thereafter, the growing chains are end-capped with MeCl and the reaction mass is filtered to remove salt. The polymer chains are then precipitated in water or MeOH, further treated to remove the residual solvent, and dried. Alternately, the solvent may be removed by flash evaporation and the reaction mass passed through a devolatizing extruder directly to remove residual solvent and for polymer granulation.
Adding more than one hydroxy monomer to the above leads to ter-polymers with three, instead of two, monomer units incorporated randomly in the chains.
It is desirable that a method of block copolymer formation is evolved whereby two plastics, both of which are sulfone-based oligomers, are connected to form a single chain as a block copolymer. As noted earlier, block copolymers comprising two or more different polysulfones are not known in the art.
In general, as is known in art, three requirements need to be met for the successful formation of block copolymers from homoblocks:                i) The two homoblocks should have end groups that react with each other, i.e. —OH & —CNO.        ii) Each homoblock should have identical end groups i.e. —OH or —CNO.        iii) The two homoblocks should be mixed in exact stoichiometric proportions in order to obtain high molecular weights.        
The present invention discloses a process of preparing block copolymers using two or more different polysulfones homoblocks and avoids the strict requirement that the individual homoblocks must have the same end groups. Similarly, it is not necessary for the two or more homoblocks used to be in equivalent stoichiometric proportions for high molecular weight block copolymer formation.